As a system for controlling a variable displacement pump in displacement, for example as shown in FIG. 6, one so controlling as to keep a difference between a delivery pressure and a load pressure constant is known.
Namely, a displacement control member 2 of a variable displacement pump 1 is actuated in a displacement reducing direction by means of a large piston 3, and actuated in a displacement increasing direction by means of a small piston 4 so that: a pressure chamber 3a of the large piston 3 is so controlled through a load sensing valve 5 as to be connected with a tank side and a delivery passage 1a; and a pressure chamber 4a of the small piston 4 is connected with the delivery passage 1a.
The above-mentioned load sensing valve 5 controls the variable displacement pump 1 in displacement in a manner such that: under the influence of a pressure or delivery pressure P.sub.1 in the delivery passage 1a of the pump 1, the load sensing valve 5 is moved into a first position I in which the pressure chamber 3a of the large piston 3 communicates with the delivery passage 1a, and, under the influence of a pressure or load pressure P.sub.LS in an outlet side of an operated valve 6, the load sensing valve 5 is moved into a second position in which the pressure chamber 3a communicates with a tank; and, when a difference (P.sub.1 -P.sub.LS) between the delivery pressure P.sub.1 and the load pressure P.sub.LS increases, the pressure in the pressure chamber 3a of the large piston 3 is increased to actuate the displacement control member 2 in the displacement reducing direction so that a delivery flow rate/revolution is reduced to lower the delivery pressure P.sub.1, and when the difference (P.sub.1 -P.sub.LS) between the delivery pressure P.sub.1 and the load pressure P.sub.LS decreases, the pressure in the pressure chamber 3a of the large piston 3 is reduced to actuate the displacement control member 2 in the displacement increasing direction so that a delivery flow rate/revolution is increased to increase the delivery pressure P.sub.1, whereby the variable displacement pump 1 is controlled in displacement so as to keep always the difference (P.sub.1 -P.sub.LS) between the delivery pressure P.sub.1 and the load pressure P.sub.LS constant.
In FIG. 6: 7 is a pressure compensating valve; and 8 is a shuttle valve for detecting a higher load pressure which is detected by the shuttle valve 8 when a plurality of the operated valves 6 are simultaneously operated, the pressure being adapted to act on the above-mentioned load sensing valve 5 and each of the pressure compensating valves 7.
In such displacement control system: since the difference between the delivery pressure P.sub.1 and the load pressure P.sub.LS is controlled so as to be constant, a delivery flow rate/unit time does not vary even when an engine 9 varies in rate of revolution, provided that the operated valves 6 are kept constant in opening.
For example, when the rate of revolution of the engine 9 decreases to cause the variable displacement pump 1 to be decreased in rate of revolution and in delivery flow rate/unit time, the difference between the delivery pressure P.sub.1 and the load pressure P.sub.LS decreases so that the displacement control member 2 is actuated in the displacement increasing direction by means of the small piston 4 so as to increase the delivery flow rate/revolution and the delivery flow rate/unit time, whereby the difference between the delivery pressure P.sub.1 and the load pressure P.sub.LS is kept constant, and, therefore the delivery flow rate/unit time does not vary even when the rate of revolution of the engine 9 varies. For example, in a condition in which the operated valves are opened to the maximums, it is kept constant as shown in FIG. 7 with A.
As a result, for example, when it is applied to a hydraulic circuit for a work unit of a construction machine, since a flow rate delivered to an actuator for the work unit is kept constant to keep the work unit constant in working speed even when the rate of revolution of the engine is changed in a condition in which the operated vales 6 are kept constant in openings, it is impossible for the work unit to perform its work with fine operations realizing a highly accurate work.
For example, even when a power shovel is used, it is impossible to perform pipe slinging operations and normal plane finishing operations too in a pipe burying work.